1. Field of the Invention
This invention relates generally to an air heating apparatus and, more particularly, to an apparatus for heating air for use in connection with a dryer for particulate material.
2. Description of the Prior Art
It is often necessary or desirable to dry freshly harvested grain before it is processed or stored. Storage of grain with excess moisture may cause quality deterioration and spoilage during subsequent storage.
The need to dry grain prior to storage has long been recognized in the art and many grain drying systems have been developed to accomplish this purpose. In many such prior systems, the grain is heated by air at a predetermined temperature during a first drying process and then the grain is quickly cooled to a desired storage temperature by exposing the grain to a flow of ambient air. One such system is the cross-flow column type grain dryer in which grain flows downwardly by gravity through a column having perforate walls and heated air is forced transversely through the perforate walls of the column to contact the grain to dry the grain or remove moisture. Typical of such cross-flow grain dryers are the grain dryers shown and described in U.S. Pat. No. 2,732,630 to Markowich and U.S. Pat. No. 3,238,640 to Fry.
The typical prior art grain dryers of the type described above generally include one or more blowers or fans which direct a flow of air through a combustion chamber having a natural gas or propane burner. The flow of air is mixed with the combustion gas from the burner to provide the heated air flow which is passed through the grain for drying. The natural or propane gas burns relatively cleanly so the grain being dried is not detrimentally affected by the inclusion of the exhaust gas within the drying air flow. While such prior art air heating systems are satisfactory in areas of the country where there is a convenient and economical supply of natural or propane gas, in other areas of the country where such gas is not readily available or is not available at reasonable prices, alternative fuels must be employed. Since other fuels, for example oil or coal, do not burn as cleanly or as completely as natural or propane gas, it is impractical to substitute these alternative fuels in such an air heating system in which the combustion gas flows through the grain being dried. The present invention provides an air heating apparatus which may be employed in connection with a grain dryer to provide either a flow of direct heated air including combustion gas or a flow of indirectly heated air which does not include such combustion gas.